1. Field of the Invention
The present invention relates to an asymmetrically coupled copolymer, a dual batch process for its manufacture and its use for the production of tire components.
2. Description of the Background
It is generally known that the properties of a rubber are determined to a significant extent by the shape of its tan delta curve. It is also known that a wide peak in the tan delta curve has a favorable effect on a series of antagonistic rubber properties which are improving the skid resistance, reducing the dynamic heat formation and lowering the rolling resistance of rubber without reducing the abrasion resistance of rubber and providing a consistent level of daily reliable use.
In the past numerous attempts have been made to develop rubbers having a high level of qualities. In order to attain as favorable as possible a compromise between the properties which in part are conflicting, mixtures of different rubbers have been employed. Another avenue which has been followed is to prepare rubber copolymers of two different blocks. Thus, copolymers are known which differ in the nature of their chemical composition and/or their structure. Examples are styrene-butadiene block copolymers, the blocks A and B of which have different styrene and/or vinyl contents. The processing of such copolymers is relatively difficult.
A further possibility to modify the properties of rubber, and in particular to improve the cold flow characteristic resides in coupling the so-called "living copolymers" present at the end of a given growing polymer strand with polyfunctional coupling agent such as a polyhalogenide or a polyalkenyl aromatic compound. Often, however, the coupling yield, i.e. the percentage of coupled products in relation to the total amount of polymerization products, is very low. According to that principle one invariably obtains coupled products, the branches of which are identical with respect to molecular weight, composition and structure. In principle coupled products are also known having different branching. However, those products invariably contain blocks composed of styrene or butadiene units. Apart from the aforegoing, thermoplastic coupling products with different branches are known. To date there has been no indication that such products can be employed in tire manufacture. For example, U.S. Pat. No. 4,248,983 describes a thermoplastic star-shaped block copolymer having the formula: (A--B/A').sub.m --X--(A'/B).sub.n, which contains 60 to 95% styrene and 40 to 5% of a conjugated diene. Residue A in the formula represents a non-elastomeric polymer segment which contains 80 to 90% styrene. A'/B and B/A' represent elastomeric polymer segments based on styrene and a conjugated diene. These products are manufactured by polymerizing the non-elastomeric segment A, adding an initiator, continuing the reaction by polymerizing a mixture of styrene and a diene and finally coupling the polymer strands. Similar block copolymers are described in U.S. Pat. Nos. 4,180,530; 4,221,884 and 4,248,980 to 4,248,984.
U.S. Pat. No. 4,391,949 describes coating compositions which are prepared from star-shaped block polymers of the formula: (A--B).sub.x Y--(C).sub.z, wherein Y represents the residue of a coupling agent, A the polymer of a monovinyl aromatic compound such as, for example, a polystyrene block, and B and C polymers of conjugated dienes, for example, polybutadiene blocks. The two polymers A--B and C are initially produced in two separate reactors. Thereafter, the contents of both reactors are combined and the coupling agent is added.
All known block copolymers are subject to at least one of the following shortcomings:
i) The block copolymers do not adequately satisfy the increasing standards of rubber properties associated with their use in tire manufacture. PA1 ii) Problems arise regarding the compatibility of the two blocks. PA1 iii) The tan delta curve exhibits only a narrow damping maximum.
A need therefore continues to exist for a rubber which exhibits an improved spectrum of properties.